Nowadays, portable electronic apparatuses with a single cell Li-ion/Polymer battery (e.g., mobile phone, MP3/MP4 player, socket charger, etc.) are popular. A USB (universal serial bus) charger which can receive power via a USB interface to charge the battery is widely used in the portable electronic apparatuses. Both USB host and AC (alternating current) adapter can serve as a power source and provide power to the USB charger. The USB charger receives the power and charges the battery. However, the USB host and the AC adapter have different interior structures and can enable the charger to charge the battery with different charging currents. In order to make better use of each power source discriminatingly, some kinds of conventional USB chargers can identify the power source and control the power to charge the battery accordingly. In this instance, two data lines are coupled between the USB host/AC adapter and the conventional USB charger. The USB charger identifies whether the power source is a USB host or an AC adapter by analyzing data received through two data lines.
As a result, two input pins are needed in the conventional USB charger to identify the type of the power source. The IC package of the conventional USB charger can be larger because of the high pin count. Also, the cost of the conventional USB charger can increase with the pin count. Furthermore, the conventional USB charger may need a costly system since data analysis through two data lines can be complicated.